Bin mode consists of grouping pixels to resolve the image. For example, the Bin 2 × 2 will group 4 contiguous pixels (the Bin 1 × 1 = 1 pixel, the Bin 3 × 3 = 9 pixels). The image will be 4 times smaller, but we will reduce the exposure time by the same factor of 4 (or we will obtain four times more signal than Bin 1 × 1 mode for the same exposure time), which is the advantage of Bin mode.
Bin 2 × 2 mode is useful for balancing the resolution of the image with the optics used (for more details, see the section astronomical calculations to section The sampling of a pixel in arc second). It is mainly used for LRGB technique with monochrome CCD cameras. The reason is that very few color CCD cameras offer the Bin 2 × 2 in color. So this solution is less available for the latter. To maintain a large format image, it is recommended to use a wide field CCD array, such as the popular Kodak KAF-8300 monochrome which is offered by a lot of CCD camera manufacturers.
It should be noted that the Bin 2 × 2 mode which makes it possible to acquire four times more signal than the Bin 1 × 1 is not accessible for cameras with CMOS sensor. This is due to the design of these dies. So here is the difference:
Matrix or CCD sensor
The digital (analog / digital) conversion takes place outside the CCD matrix, allowing the pixel size to be dynamically changed using Binning. For example, the Bin 2 × 2 will allow to group four adjacent pixels allowing to acquire 4 times more signal than the Bin 1 × 1 (a single pixel).
CMOS matrix
Each pixel is an element wired to its own read circuit. This means that there is no real on-chip binning. The digital conversion is carried out pixel by pixel, not allowing to acquire 4 times more signal than the Bin 1 × 1 (a single pixel). The 2 × 2 bin of the CMOS matrix provides more images per second. This mode is therefore more interesting for the photography of planets. This gives more images per second. The benefit for the deep sky is very minimal, considering that the longer transfer time for the CCD arrays is not an important element for the deep sky.
Richard Beauregard
Sky Astro - CCD
Revised 2021/01/04
Reference for technical differences between CCD and CMOS matrices:
https://www.atik-cameras.com/news/difference-between-ccd-cmos-sensors/